Iris-loaded slow wave guide for microwave linear electron accelerator having irises differently oriented to suppress unwanted modes



Oct. 20, 1964 R. L. KYHL 3,153,767

I LOADED SLOW WAVE GUIDE FOR MICROWAVE LI R HAV TRON ACCELERATOR ING IRI DIFFEREN ORIENTE O SUPPRE UNWA D MODES led June 1960 1 United States Patent 3,153,767 IRIS-LOADED SLOW WAVE GUIDE FOR MICRO- WAVE LINEAR ELECTRON ACCELERATGR HAVING RISES DIFFERENTLY ORENTED T0 SUFPRESS UNWANTED MODES Robert L. Kyhl, 46 Hickory Cliff Road, Newton, Mass. Filed June 13, 1960, Ser. No. 35,739 1 Claim. (Cl. 33331) This invention relates to microwave linear electron accelerators. In such accelerators, for a given RF power input, the electron beam current may theoretically be increased at the expense of electron beam energy. In prac tice, however, the maximum theoretical current has not been achieved and the electron beam current has been limited by a phenomenon which has been designated beam blow-up. The object of the invention is to prevent beam blow-up by appropriate modification of the accelerating waveguide structure.

In a typical microwave linear electron accelerator the TM mode of round wave guide is used to accelerate the electrons. I believe that beam blow-up is caused by lateral deflection of electrons in the beam by the RF field in other modes, particularly TM modes. More specifically, I believe that beam blow-up is caused by the accelerator oscillating as a backward Wave oscillator of the transverse type in the TM mode. The TM mode is a likely cause of beam blow-up because it is one of the first higher modes to be found: in a 2600 megacycle accelerator the TM mode will fall in the 4000-()O0 megacycle range. Moreover, the TM mode provides a field for deflecting the beam, since it is a transverse-H mode. Again, the TM mode has a longitudinal E field and can in principle extract energy from the beam. And, finally, it is a backward wave and hence subject to backward wave oscillations.

This mode has two planes of polarization. In accordance with the invention, by spoiling the axial symmetry of the usual accelerator structure, the polarization degeneracy is removed. Axial symmetry is spoiled by treating alternate cavities oppositely, so as to decouple successive cavities as far as the TMIIQ mode is concerned. This mode now has two separate resonant frequencies, but the important thing is that the group velocity for this mode is reduced practically to zero, which thus impairs the backward wave oscillations or prevents them. Anything which spoils the axial symmetry will have an effect on deflection-type mechanisms of any sort.

In another modification of the invention, alternate successive cavities of the accelerator structure are detuned for the TM mode while leaving the TM mode unperturbed.

Both embodiments of the invention achieve the same objective: namely, to detune successive cavities for those modes which by symmetry would normally be coupled.

In still another modification of the invention, absorbing material is used to preferentially damp out the undesired mode. Currents in the TM or electron-accelerating mode are all radial in the disks of the waveguide, while the TM mode has angular currents. Thus, in accordance with the invention, I provide resistive material in radial slots in at least some of the disks to preferentially damp out the undesired TM mode while leaving the desired TM mode relatively unaffected.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawings in which:

FIG. 1 is a somewhat diagrammatic view partly in side elevation and partly in longitudinal central section of an iris-loaded waveguide for a microwave linear accelerator incorporating one embodiment of the invention;

FIG. 2 is a view along the line 22 of FIG. 1;

3,153,767. Patented Oct. 20, 1964 lCC.

FIG. 3 is a view along the line 33 of FIG. 1;

FIG. 4 is a view similar to that of FIG. 1 and showing a second embodiment of the invention;

FIG. 5 is a view along the line 5-5 of FIG. 4;

FIG. 6 is a view similar to that of FIG. 1 showing a third embodiment of the invention;

FIG. 7 is a view along the line 7-7 of FIG. 6;

FIG. 8 is a view similar to that of FIG. 1 showing a fourth embodiment of the invention;

FIG. 9 is a view along the line 9-9 of FIG. 8;

FIG. 10 is a view similar to that of FIG. 1 showing a fifth embodiment of the invention; and

FIG. 11 is a View along the line 1111 of FIG. 10.

Referring to the drawings and first to FIGS. 1, 2 and 3 thereof, therein is shown one structure in which alternate cavities of an iris-loaded Waveguide are treated oppositely. All the cavities of the iris-loaded waveguide 1 shown in FIG. 1 are of elliptical cross-section, but the orientation of these ellipses alternates from one cavity to the next. The effect is to spoil the propagation of the unwanted mode without spoiling the propagation in the fundamental mode which is the mode used in microwave linear accelerators for electron acceleration. Since all of the unwanted modes have polarization, it is possible to detune successive cavities for the undesired modes but not detune them for the desired modes.

The embodiment of the invention shown in FIGS. 1, 2 and 3 involves certain practical constructional difficulties and in general it will be better to keep the circular geometry of the conventional accelerating waveguide and spoil the axial symmetry by inserting conductive slugs on one or both sides of the disks. Thus, referring to FIGS. 4 and 5, every other disk 2 has a metal slug 3 brazed solidly onto the disk 2 or otherwise made integral with the structure of the disk 2.. As shown in FIGS. 4 and 5, two slugs 3 may be used and preferably are placed at the point of least tuning for the proper mode or electron accelerating mode for precision purposes. Two slugs 3 are preferable to one slug, since the asymmetry introduced in the desired mode by a single slug might cause undesirable effects on the electron beam. The intervening disks 4 are similarly provided each with two conductive slugs 5. These slugs however are displaced with respect to the slugs 3.

As indicated hereinbefore in another modification of the invention, alternate successive cavities are detuned for the unwanted mode While leaving the wanted mode unperturbed. Referring to FIGS. 6 and 7, every other disk 6 is provided with four conductive slugs 7 which are placed at the zero tuning of the wanted mode which in the case of the TM mode is at a point about half way out along the radius. Zero tuning results when the effect on resonant frequency of the slug material in the electricfield regions balances the effect on resonant frequency of the slug material in the magnetic-field regions. Unlike the embodiment shown in FIGS. 4 and 5 in the embodiment of FIGS. 6 and 7, the arrangement of the slugs 7 on the disks 6 may show axial symmetry, but in this case it is necessary that the intervening disks 8 have no such conductive slugs. The number of slugs is not important and indeed as shown in the embodiment of FIGS. 8 and 9, the slugs may be replaced by a conductive ring 9 placed at the zero tuning of the wanted mode on alternate disks 10, the intervening disks 11 being of the conventional type without any protuberancy on its surface which forms the boundary of the cavities.

As previously pointed out, where as is usually the case, the currents in the wanted mode are all radial in the disks while the unwanted mode has angular currents, the arrangement shown in FIGS. 10 and 11 may be employed.

Referring thereto, some or all of the disks 12 are provided with resistive material 13 arranged in radial slots.

This resistive material 13 will absorb energy from the angular currents of the unwanted mode but will not absorb energy from the radial currents in the wanted mode.

Having thus described the principles of the invention together With several illustrative embodiments thereof, it is clearly to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claim.

I claim:

An iris-loaded waveguide adapted to propagate an electromagnetic Wave including the TM mode for use in a microwave linear accelerator, which iris-loaded Waveguide includes a multiplicity of cavities, a first group of said cavities having an elliptical cross-section with common orientation and a second group of said cavities having an elliptical cross-section with common orientation difierent from that of said first group, the cavities of said second group being interspersed between the cavities of said f; first group, said first and second groups of cavities being adapted to detune said Waveguide for extraneous modes of said electromagnetic Wave which by symmetry would normally be coupled while having substantially no eifect upon the TM mode of said electromagnetic wave.

References Cited in the file of this patent UNITED STATES PATENTS 2,398,162 Sloan Apr. 9, 1946 2,632,804 Joguet Mar. 24, 1953 2,691,766 Clapp Oct. 12, 1954 2,698,923 Edson Jan. 4, 1955 2,710,945 Edson June 14, 1955 2,891,225 Lewis et a1. June 16, 1959 2,892,958 Nygard June 30, 1959 2,899,598 Gunzton Aug. 11, 1959 2,939,993 Zublin et al. June 7, 1960 2,952,795 Craig et al. Sept. 13, 1960 2,953,708 Duncan Sept. 20, 1960 

